Permanent casting moulds are generally multi-part and in each case comprise at least one mould body, which delimits, at least in sections, the mould cavity reproducing the cast part to be produced. In this case, the mould body is typically manufactured from a highly heat-resistant tool steel, which, despite the high mechanical and thermal stresses occurring during the casting operation, ensures an adequately long service life of the mould body.
Because of the high quality demands, (in respect of the contour accuracy of the cast products) the production of permanent casting moulds of the type in question is laborious and expensive. The aim is therefore to use permanent casting moulds for as long as possible and for as many parts as possible. This applies, in particular, to permanent casting moulds, which are used for high volume production of cylinder heads, as the production of such moulds is particularly intensive in terms of work and costs because of the complexity of shaping such cylinder heads.
In order to be able to vary certain shaped elements of the cast part to be produced, without having to produce a completely new casting mould for this purpose in each case, permanent casting moulds are generally equipped with casting mould inserts, which are inserted into the interior surrounded by the permanent casting mould and reproduce the respectively desired shaped elements, such as recesses or elevations in the cast part to be produced, in each case.
A typical example of the use of inserts of this type emerges in the production by casting of cylinder heads for internal-combustion engines already mentioned at the outset. The output yield, the combustion behaviour and the consumption of a combustion engine associated therewith is decisively influenced by the shape of the combustion chambers formed in each case in the cylinder head, into which the respective fuel is let via at least one inlet valve and from which the exhaust gases are expelled via at least one outlet valve.
In order to be able to modify the combustion chamber shape in a simple manner in a certain basic type of cylinder head, permanent mould inserts are generally inserted in the permanent casting mould provided for casting this cylinder head, the surface of which inserts associated with the mould cavity surrounded by the mould determines the shape of the combustion chamber recesses to be produced in each case in the cylinder head. Casting mould inserts of this type are also designated “combustion chamber inserts” in the technical language. For this purpose, they are seated in the receivers formed in the walls of the mould body delimiting the mould cavity.
During the casting process, because of the contact of the casting mould with the molten metal cast in each case, strong heating of the respective mould body and the casting mould inserts seated therein occurs. Because of this heating, the mould body and casting mould inserts expand. The extent of this expansion depends, on the one hand, on the temperature increase occurring in the mould body and casting mould insert, in each case, and, on the other hand, on the expansion behaviour of the material respectively used for the production thereof. Thus, there is generally a varying degree of expansion of the mould and casting mould insert, because the masses of the inserts and of the mould body are different, so the casting mould insert having a substantially lower mass heats up very much more quickly than the mould body surrounding it with the result that the insert expands more quickly and to a greater extent than the mould body surrounding it. This phenomenon does not only occur when the mould body and casting mould insert consist of different materials, but because of the lesser volumes, even when they are made from the same material.
The different expansion behaviour of the casting mould inserts and mould bodies leads to dimensional imprecisions, which prove to be particularly difficult to manage, for example, when casting is to take place so as to be as close as possible to final dimensions. This requirement proves to be particularly critical in the high volume manufacturing of cylinder heads, for which a maximum permissible dimensional variation of +/−0.15 mm relative to the desired dimension is required by the engine producer.
In order to ensure the reliable processing of the respectively required accuracy of the dimensional stability even under these conditions, complex measures for the installation of the casting mould inserts and a temperature distribution as uniform as possible in the mould body are required. Thus, in known permanent casting moulds used to cast cylinder heads, the position of the casting mould inserts in the respective mould body generally firstly has to be determined empirically by a large number of tests in order to compensate the heat expansion occurring during the respective casting process by a corresponding dimensioning of the casting form insert in such a way that an adequately precise casting result is achieved. If a suitable dimensioning is not possible in the framework of the installation space available in each case or with regard to the functioning and stability of the respective casting mould insert, cooling of the casting mould inserts may become necessary to reduce the heat expansion.
As, despite all the outlay effected to determine an optimal shape of the casting mould insert, unacceptable dimensional deviations frequently still occur in the casting operation when using conventional moulds, a check of the respectively completed heads for adherence to the required depth of the combustion chambers to be respectively formed in them generally has to be carried out in large volume manufacture by casting of cylinder heads, for example.
The attempt is known from DE 198 38 561 A1, to increase the service lives of moulds, in which non-iron metals, such as magnesium melts are cast into cast parts, in that the mould bodies and the casting mould inserts inserted therein are produced from a heavy metal material with a high melting point, such as molybdenum or tungsten. The advantage of using such materials for the production of casting moulds is seen in that they are attacked less by the respective molten light metal than conventional steels and are subject to accordingly lower corrosion.
Regardless of this advantage of the material selection proposed in DE 198 38 561 A1, the problem, however, exists that even the mould bodies manufactured from heavy metals with a high melting point and the casting mould inserts inserted therein expand to a different extent during heating because of their respective different volume and their faces of different sizes coming into contact with the melt. Thus, dimensional deviations caused by the different heat expansion are noticeable in practice even when heavy metal materials, which have particularly low coefficients of thermal expansion, are used according to the model of DE 198 38 561 A1.